In conjunction with the recent rapid advances of portable electronic equipment and communications instruments, nonaqueous electrolyte secondary batteries having a high energy density are strongly demanded from the aspects of cost, size and weight reductions. On the other hand, in the automotive application, active efforts are made on the development of hybrid cars and electric vehicles for the purposes of improving fuel consumption and suppressing the emission of global warming gas.
Silicon is regarded most promising in attaining the battery's goals of size reduction and capacity enhancement since it exhibits an extraordinarily high theoretical capacity of 4,200 mAh/g as compared with the theoretical capacity 372 mAh/g of carbon materials that are currently used in commercial batteries.
For example, JP 2964732 discloses a lithium ion secondary battery using single crystal silicon as a support for negative electrode active material. JP 3079343 discloses a lithium ion secondary battery using a lithium alloy LixSi (0≤x≤5) with single crystal, polycrystalline or amorphous silicon. Of these, the lithium alloy LixSi with amorphous silicon is preferred, which is prepared by coating crystalline silicon with amorphous silicon resulting from plasma decomposition of monosilane, followed by grinding. Although the amount of a silicon component used is as small as 30 parts as described in Example, the material fails to exhibit cycle stability over several thousands of cycles as achieved by graphite-based materials. Thus the material has never been used in practice.
JP 3702223, 3702224 and 4183488 disclose deposition of an amorphous silicon thin film on an electrode collector by evaporation method, and use of the resulting electrode as a negative electrode. In conjunction with this direct gas phase growth of silicon on the current collector, JP-A 2006-338996 discloses to control the growth direction for suppressing a lowering of cycle performance due to volume expansion. Although this method is successful in improving cycle performance, there are still left problems that the cost is high because the electrode manufacture speed is limited, it is difficult to increase the thickness of silicon thin film, and copper used as the negative electrode collector diffuses into silicon.
Recent approaches taken for this reason include a method for restraining volume expansion by restricting the percent utilization of silicon battery capacity using silicon-containing particles (JP-A 2000-173596, JP 3291260, and JP-A 2005-317309), a method of quenching a silicon melt having alumina added thereto for utilizing grain boundaries in polycrystalline particles as the buffer to volumetric changes (JP-A 2003-109590), polycrystalline particles of mixed phase polycrystals of α- and β-FeSi2 (JP-A 2004-185991), and hot plastic working of a monocrystalline silicon ingot (JP-A 2004-303593).
As discussed above, metallic silicon and silicon alloys having various crystal structures have been proposed to utilize silicon as the active material. They fail to display cycle stability comparable to graphite and require increased costs. That is, a material ensuring economical, large-scale synthesis is not available.
The negative electrode shaped form is prepared by mixing a negative electrode active material, coating to a current collector, drying, pressing and cutting to the desired size. Heretofore, graphite is generally used as the negative electrode active material since it has relatively high fluidity and acceptable dispersibility. As to silicon and silicon compound particles of recent interest, since these particles are strongly adhesive, the powder is not amenable to continuous supply because metering errors often occur when the powder is supplied by a feeder. When an electrode slurry is prepared, the particles are likely to agglomerate. The resulting negative electrode shaped forms tend to be non-uniform.